This invention relates to seismic roll along switching equipment and in particular to an improved electronically controlled switching system having a greatly reduced number of signal switches.
In seismic geophysical prospecting the geophysical roll along approach is commonly employed. In this method a large number of geophone groups and shot points are laid out along a prospecting traverse. During any given initiation, or shot, only part of the geophones are used to record reflections from the initiation. For example, in a typical system 96 geophone groups may be laid out while only forty eight, twenty four on either side of the shot point, will be used to record reflections from the particular shot point. Such systems typically have one signal processing channel for each of the active geophone groups. Each processing channel comprises amplifiers and filters which are matched to a very high precision and typically comprise the most expensive single part of the prospecting system.
The outputs of the signal processing channels are usually coupled by means of a multiplexer to the input of a single analog to digital converter, the output of which is serially recorded on magnetic tape for later processing. It is generally preferred that the channel outputs be recorded in a sequence corresponding to the positions of the active geophone groups starting at one end of the spread. It is apparent that a rather complicated switching matrix is required to connect a particular set of 48 geophones out of the 96 groups to the processing channels and to change this for each of the shot points provided on the traverse. For example, at the first shot point on the traverse it may be desired to connect geophones number 1 through 48 to the forty eight amplifier channels in sequence. At the second shot point in the roll along process geophone groups numbered 2 through 49 may then be connected in order to the same forty eight processing channels. When carried through the entire seismic traverse, it can be seen that each of the amplifier inputs must be capable of being connected to any one of at least forty nine geophone group outputs. When the forty eight inputs are multiplied by the forty nine possible positions it can be seen that a total of 1,352 switches must be used to accomplish this goal. Since each geophone output provides a low signal level, each of the lines has its own ground line so that the total number of switches is actually doubled.
The presently used commercial roll along switches do in fact use either mechanical relays or switches having literally thousands of switch contacts for performing the roll along function. Some commonly used switching matrixes employ large numbers of sliding switch contacts which are moved across the surface of a printed circuit board to provide the necessary sets of connections. A rotary version of this type matrix is sold under the trademark "Rot-A-Long" by Input/Output Devices, Inc., of Houston, Tex. and a linear version is sold under the part number S100-002131 by GUS Manufacturing, Inc., of El Paso, Tex.
The known roll along matrixes are limited in several ways by the fact that they are basically mechanical devices. The thousands of switch contacts which are employed are prone to contact failure especially when used in field conditions typically encountered in geophysical prospecting. The devices tend to to be quite inflexible since a large number of switches must be arranged to provide at least one common set of roll along positions and in most practical cases only the one set can be performed by a given switching matrix. While solenoid and ratchet arrangements can be used to remotely move the known roll along matrixes through their various positions, any failure of the mechanism to operate will cause erroneous selections of groups throughout the rest of the operation. As a result of these problems the roll along matrix has essentially always been housed in the main recording equipment near an operator who is able to visually check for proper operation.
It can be seen that a reduction in the number of switches involved in a roll along switching matrix is desirable and that any substantial reduction in the number would greatly enhance the accuracy and reliability of geophysical prospecting systems. In addition it is desirable that the switches employed in such systems be electronically switchable to avoid mechanical failures and to increase the flexibility of operation.